Stable inheritance of an organism's genetic complement is required for continuity of the species and normal development of the individual. An elementary partition mechanism for ensuring genetic continuity is encoded by the bacterial plasmid P1. Its functional analysis is the main topic of our research. The P1 partition module consists of two structural genes and a cis-acting DNA site (the plasmid centromere) that together distribute P1 plasmids to daughter cells such that no cell is deprived of a plasmid copy. Similar partition modules are found throughout the two bacterial kingdoms, in plasmids and chromosomes as well. This laboratory uses methods of genetics, molecular biology and fluorescence microscopy to study the dynamic localization along DNA and within the cell of each component of the partition apparatus and thereby elucidate the partitioning mechanism. Previously we reported the striking capacity of the P1 centromere-binding protein, ParB, to spread along DNA flanking the centromere, silencing genes in its path (1). The relevance of the ParB-centromere interaction to regulation of partition gene expression (2) and of ParB spreading to the overall partition process have been studied during the period of this report. In this same period we have published evidence that ParB exhibits a centromere-pairing function, a likely first step in partitioning (3). Genetic and physiological influences on the cellular localization of ParA, an ATPase with a presumed dynamic role in partitioning, are currently under study. Finally, the P1, in collaboration with two former postdoctoral fellows, is contributing to an extensive analysis of the P1 genome (4).